1. Field of the Invention
The present invention relates to a thin film transistor array substrate in which a transmissive pixel electrode and a reflective pixel electrode are disposed in each pixel and a storage capacitor electrode is disposed below the reflective pixel electrode, a method of manufacturing the same, and a transflective liquid crystal display using the same.
2. Description of Related Art
It is normally necessary for the manufacturing of a transflective liquid crystal display (LCD) to form a transmissive pixel electrode and a reflective pixel electrode. This causes an increase in the manufacturing process for a thin film transistor (TFT) array substrate. To overcome this drawback, a method of forming a reflective pixel electrode at the same time as forming a source-drain electrode from the same layer is used as described in Japanese Unexamined Patent Publication No. 2005-292660, for example.
In the case of using a source-drain electrode as a reflective pixel electrode, contact resistance between Al metal having a high reflectance and ITO used for a transmissive pixel electrode in an upper layer can be undesirably high. To avoid this, there is proposed a technique of forming the source-drain electrode in a two-layer structure composed of Al metal and Cr metal and implementing two-time exposure, halftone exposure or gray-tone exposure (referred to hereinafter as two-stage exposure) to thereby remove the Al metal in the upper layer only in the portion to be in contact with ITO.
In such a method, a resist thickness varies according to a pattern formed in a gate electrode process as an under layer of the resist, i.e. the resist thickness is small in the portion where the pattern is formed and the resist thickness is large in the portion where the pattern is not formed. The resist thickness after performing two-stage exposure varies accordingly, which causes a decrease in accuracy of the two-stage exposure. In order to allow the thickness of the resist where two-phase exposure is performed to be as uniform as possible, it is preferred to uniform the film structure of the under layer of the resist in the portion where light exposure is implemented. In the case of performing two-stage exposure on the source-drain electrode, it is preferred to leave the pattern of the gate electrode process which serves as the under layer of the light-exposed portion. For these reasons, a storage capacitor electrode 1 which is formed in the gate electrode process is placed below a contact hole 2 which is created by the two-stage exposure as shown in FIGS. 9 and 10.
However, this structure causes dielectric breakdown to occur in the gate insulation layer 4 between a reflective pixel electrode 3 and the storage capacitor electrode 1 under the contact hole 2. The dielectric breakdown tends to occur in the opening of the contact hole 2. One reason for this is as follows. A pinhole is formed in the two-phase exposed part of the resist due to the exposure energy or the resist thickness being nonuniform, and the reflective pixel electrode in this part is etched off. As a result, the insulation layer 4 is hollowed out in the contact hole formation process, causing short-circuit to occur between the transmissive pixel electrode 5 and the storage capacitor electrode 1. The storage capacitor electrode 1 and the transmissive pixel electrode 5 are thereby electrically short-circuited through the reflective pixel electrode 3, which is recognized as a luminescent spot. The luminescent spot is a major factor for reducing process yield of a liquid crystal display.
Further, if the storage capacitor electrode 1 under the contact hole 2 is hollowed out, a cell gap can be large in some portions of the reflective pixel electrode of the transflective LCD, which results in degradation in reflective optical properties.